Drive control apparatus for an injection molding machine

Abstract

A drive control apparatus connects a hydraulic source 4 and an oil tank 5 to an injection cylinder 2 through a servo valve 3 and controls the injection cylinder 2 by the servo valve 3. A first change-over valve 6 which is connected between the hydraulic source 4 and the servo valve 3 and has a large capacity. A flow limiting circuit 9 connects a throttle valve 7 to a second change-over valve 8 having a small capacity in series and is connected to the first change-over valve 6 in parallel. A change-over valve control portion 10 opens both the first change-over valve 6 and the second change-over valve 8 when controlling a large rate of flow in an injection process or the like and controls to close the first change-over valve 6 when controlling a small rate of flow in a pressure maintenance process or the like. As a result, the first change-over valve 6 having a large capacity and the second change-over valve 8 (which is connected to the throttle valve 7 in series and has a small capacity) are opened so that the control suitable for the large rate of flow is performed when controlling the large rate of flow. When the first change-over valve 6 having a large capacity is closed, control of a small rate of flow is carried out.

What is claimed is: 
     
       1. A drive control apparatus for an injection molding machine in which a hydraulic source and an oil tank are connected to an injection cylinder through a servo valve and the injection cylinder is controlled by the servo valve, the drive control apparatus for an injection molding machine comprising: a first change-over valve which is connected between the hydraulic source and the servo valve and has a first capacity of hydraulic fluid; a flow limiting circuit which connects a throttle valve to a second change-over valve having a second capacity of hydraulic fluid smaller than that of the first change-over valve in series and is connected to the first change-over valve in parallel; and   a change-over valve control portion which controls to open both the first change-over valve and the second change-over valve when controlling a first rate of flow of hydraulic fluid and which controls to close the first change-over valve when controlling a second rate of flow of hydraulic fluid, said second rate being smaller than said first rate.   
     
     
       2. The drive control apparatus for an injection molding machine according to claim 1, wherein a four-port servo valve is used as the servo valve; a first port and a second port in the four-port servo valve are connected to a front oil chamber and a rear oil chamber of the injection cylinder, respectively; a third port of the servo valve is connected to the oil tank; and fourth port of the servo valve is connected to at least one of the first change-over valve, the second change-over valve and the throttle valve. 
     
     
       3. The drive control apparatus for an injection molding machine according to claim 1, wherein an injection molding process is effected when controlling said first rate of flow of said hydraulic fluid. 
     
     
       4. The drive control apparatus for an injection molding machine according to claim 1, wherein a pressure maintenance process is effected when controlling said second rate of flow of said hydraulic fluid. 
     
     
       5. The drive control apparatus for an injection molding machine according to claim 1, further comprising a change-over valve operation limiting circuit for gradually closing the first change-over valve when changing over from the control for said first rate of flow to the control for said second rate of flow of said hydraulic fluid. 
     
     
       6. The drive control apparatus for an injection molding machine according to claim 5, wherein the change-over valve operation limiting circuit connects a pilot circuit portion of the first change-over valve to the oil tank via the throttle valve when changing over from the control for said first rate of flow to the control for said second rate of flow of said hydraulic fluid.